Hemophilia A is an X-linked recessive bleeding disorder caused by the deficiency/abnormality of Factor VIII. Currently, treatment of hemophilia involves factor replacement using fresh frozen plasma, cryoprecipitate, or Factor VIII concentrate. While this greatly improves the quality of life of hemophiliacs, it is less than ideal, since regular treatments are required throughout the life of the patient. Treating hemophilia in utero would enable correction prior to disease onset, thus allowing the birth of a healthy baby who requires no further treatment. Our laboratory has developed and optimized a unique sheep model for in utero stem cell transplantation (IUSCT) that allows the engraftment and differentiation of human stem cells in the absence of preconditioning, by virtue of the early gestational recipient's pre-immune status. This model has proven to be an accurate and valuable pre-clinical system for evaluating approaches to IUSCT, and the data generated in this model was used to conduct the first successful clinical IUSCT in a patient with X-SCID. In addition to the durable hematopoietic engraftment, transplanted hematopoietic stem cells (HSC) and marrow-derived mesenchymal stem cells (MSC) also give rise to other tissues in this model, including significant numbers of functional hepatocytes. These results suggest that adult marrow-derived stem cells may be ideally suited for cellular therapy to correct disorders such as the hemophilias in which a liver-derived factor is defective or absent. In the present proposal we will test this hypothesis by utilizing cryopreserved semen to re-establish a line of sheep that exhibited spontaneous factor VIII deficiency with symptomology closely mimicking that of human hemophilia A, while simultaneously transplanting normal sheep fetuses with adult HSC and MSC to determine the optimal stem cell population(s) for generating functional hepatic cells in vivo. We will then transplant the affected fetuses in utero with the optimal stem cell population(s) and assess whether this in utero cell therapy approach produces therapeutic benefit in this clinically relevant large animal model of hemophilia A. We will also examine the liver and other tissues of the recipient hemophilic sheep to establish a correlation between the degree of clinical improvement and the levels of Factor VIII-producing cells generated by the transplanted adult human HSC and MSC. It is hoped that these studies using adult BM-derived stem cells will lead to the development of a successful stem cell-based therapeutic approach to treat hemophilia prior to birth, thus obviating the need for lifelong factor therapy with its inherent risks/shortcomings.